American Theater Company’s world premiere of the revised version of “columbinus” is a powerful documentary-style performance piece and a call to step up efforts to stop the horrible violence plaguing our schools and society. While it never fully answers the all-important question of why Eric Harris and Dylan Klebold, armed to the hilt, walked into Columbine High School in Littleton, Colo., on April 20, 1999, and murdered 12 of their fellow students and one teacher, it does provide insights into the events surrounding the tragedy.

Conceived by ATC Artistic Director P.J. Paparelli and written by him and Stephen Karam based on interviews, the play was developed under the moniker of the United States Theatre Project and originally performed as a two act in Washington, D.C., and Juneau, Alaska, in 2005. It premiered at the New York Theater Workshop in 2006, and productions all over the country followed.

The new work adds a third act crafted from more than 100 hours of interviews Paparelli did last year in Littleton, after the horrendous movie theater shooting in nearby Aurora, Colo. It examines the aftermath of the Columbine massacre at various intervals (a day, week, month, year, etc.), illuminating the short and long-term effects on witnesses, the injured, the accused and their families.

The before-during-after format of the three acts makes perfect sense, but the first act —drawn from talks with suburban teenagers in several cities — initially was intended to offer a broader picture of American adolescence and high-school life rather than concentrating just on Columbine. That’s obvious in the opening, which runs through the activities of an average school day, and in the showcase of various student types: the preppie, the perfect girl, the jock, the rebel, the freak, the loner. The last two eventually morph into Harris (Matthew Bausone) and Klebold (Eric Folks), but an argument could be made for trimming this material and focusing more on the specific instead of the generic.

The most compelling “before” segments show how the grownups — parents, neighbors, teachers, guidance counselors — failed to recognize the risk Harris and Klebold represented and/or to deal with them in any meaningful way. Twenty-twenty hindsight may not be especially useful, but I found the clueless counselors’ lack of understanding and inability to reach these kids especially disturbing.

The shocking centerpiece of “columbinus,” however, is the second act, practically an hour-by-hour account of April 20, starting with the killers’ preparations and perspective and moving to the debacle at the school. Punctuated by ear-splitting gunshot-like raps on the blackboard running across the back of the stage (sound design by Martin Desjardins and Andre Pluess), it left the opening night audience in stunned silence.

The new third act is no match in terms of dramatic impact, but it does offer more context, some closure and perhaps a sense of catharsis. On the other hand, it comes across as simultaneously schematic and a little scattered, so like Act I, it might benefit from judicious cutting and shaping.

The entire ensemble — Rob Fenton, Jerod Haynes, Kelly O’Sullivan, Leah Raidt, Tyler Ravelson and Sadieh Rifai, as well as Bausone and Folks — is outstanding, but a special nod goes to Folks for making us believe the deeply troubled Klebold could have been different if only …

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